1. Field of the Invention
The present invention relates to an audio compression system, and more particularly, to an audio compression method using wavelet packet transform (WPT) in MPEG1 layer 3 (hereinafter referred to as “MP3”) and a system thereof. The present application is based on Korean Patent Application No. 2002-8305, which is incorporated herein by reference.
2. Description of the Related Art
Generally, in an MPEG standard method, monaural audio is encoded at the rate of 128 kbps, while a layered algorithm is used to encode stereo audio at the rates of 192 kbps, 92 kbps, and 64 kbps. In the layers, layer 3 is known as an MP3 technology. The MP3 technology increases the resolution of a frequency domain by adding a modified DCT (MDCT) operation, and, by considering input characteristics in the MCDT operation, adjusts the size of a window so that pre-echo and aliasing are compensated for.
FIG. 1 is a flowchart showing a conventional audio compression method using MP3 technology.
First, pulse code modulation (PCM)-type audio data is input in step 110.
Then, PCM audio data is divided into 576 samples in each granule.
By applying a psychoacoustic model defined in the MPEG1 layer 3 to the samples, perceptual energy is obtained in step 120.
Next, the perceptual energy obtained from the psychoacoustic model is compared with a threshold, and according to the comparison result, MDCT is performed with switching windows in step 130. Here, a part of the MDCT window or the entire MDCT window may be switched according to the threshold. That is, as shown in FIG. 2, if the level of the perceptual energy is higher than the threshold, this corresponds to an attack state signal, whose energy level rapidly increases, and therefore a short window is selected. If the level of the perceptual energy is lower than the threshold, this corresponds to a constant state signal, and therefore a long window is selected. Accordingly, audio samples in the respective selected window scopes are MCDT-processed and converted into data in frequency domains. At this time, a start window or a stop window is used to switch from the long window to the short window.
Also, in the MPEG1 layer 3, the types of windowing are disclosed as a long window, a start window, a short window, and a stop window, as shown in FIG. 3. Also, as shown in FIG. 2, the windows overlap each other in order to prevent aliasing.
Then, data on the frequency domain for which MDCT is performed are quantized according to the number of assigned bits in step 140.
The quantized data is formed as a bit stream based on a Huffman coding method in step 150.
Therefore, as shown in FIG. 1, the prior art audio signal compression method uses the MDCT window switching method to compress a non-stationary signal which causes a pre-echo effect. However, the prior art audio compression method using the MDCT as shown in FIG. 1 degrades sound quality of low bit rates, less than, for example, 128 kbps (64 kbps, stereo), due to the limit of the MDCT base.